1. Field of the Invention
The present invention relates to a sensor for measuring the temperature of a fluid such as an exhaust gas of an engine, and more particularly, to a sensor in which a sensor element such as a thermistor (hereinafter, referred to as a sensor element or simply an element) is disposed in an interior of a tube which is closed at a front end thereof (a bottomed tube or a cap) at a location lying at or close to the front end of the tube. The sensor is mounted at a mounting target location such as an exhaust manifold (an exhaust pipe) such that a location lying close to the front end of the tube is exposed to heat of the exhaust gas. In this manner, the heat is conducted to the sensor element to suitably measure (detect) a temperature of the exhaust gas.
2. Description of the Related Art
Conventionally, sensors having various constructions have been proposed as sensors of this type (temperature sensors). Patent Literature 1 discloses a sensor in which a hollow bolt-like (annular or cylindrical) annular mounting fixture, which has a mounting portion (thread) formed on an outer circumferential surface thereof, is fixedly fitted on a tube (metallic case) housing a sensor element in order to mount the sensor at a mounting target location. This sensor is screwed into a threaded hole which is provided in the mounting target location for use in measuring the temperature of an exhaust gas.
In the above-described sensor, an inner circumferential surface of a tube insertion hole (a central through hole) formed inside the annular mounting fixture and an outer circumferential surface of the tube need to be fixed (connected) together structurally. In addition, a gastight seal is needed in a front-to-rear direction between the inner and outer circumferential surfaces. As a conventional sealing means, a tube is inserted into a tube insertion hole in an annular mounting fixture in a loose-fit fashion. Then, a molten brazing filler (Ni brazing filler or copper brazing filler) is caused to flow into a gap defined between an inner circumferential surface of the tube insertion hole and an outer circumferential surface of the tube so as to set and establish gastightness therebetween.
On the other hand, in a sensor of this type, in fixing together the annular mounting fixture and the tube by means of brazing with the tube inserted in the annular mounting fixture, from the viewpoint of positioning accuracy or detection accuracy that is taken into account when mounting the sensor at the mounting target location such as the exhaust manifold, high dimensional accuracy is required for a forward projection amount of the tube which projects from the annular mounting fixture. In particular, the tube needs to be mounted so that a dimension from the tip or front end of the tube to a predetermined position on the annular mounting fixture (or a projecting length from a tip or front end of the fixture) becomes highly accurate. On the other hand, in brazing together the inner circumferential surface of the tube insertion hole (the central through hole) in the annular mounting fixture and the outer circumferential surface of the tube which is inserted in the inner circumferential surface to which the tube is fixed through brazing, a molten brazing filler is caused to flow into the gap between the two members (the inner and outer circumferential surfaces thereof) by making use of capillary action, etc., so as to set (solidify) therein. The specific method used is as follows.
As shown in FIG. 19, an annular mounting fixture 31 is supported at a location on an outer circumference of the annular mounting fixture 31 which constitutes a tip or front end-facing surface (for example, a tip or front end-facing surface of a screwing tool engagement portion (a polygonal portion 34) by a support portion 215 of a support (holding) jig 211 which is provided on an upper surface 201 of a positioning table. Then, a tube 11 is inserted into a tube insertion hole 32 in the annular mounting fixture 31 from above with a tip or front end 12 of the tube 11 oriented downwards to contact the upper surface 201. By so doing, in a front-to-rear (longitudinal) direction of the tube 11, the tube 11 is positioned so that a dimension L1 from the front end 12 to a predetermined position on the annular mounting fixture 31 is held at a predetermined dimensional accuracy. Following this, with the tube 11 so positioned, a brazing filler material 220 in the form of a foil or ring is disposed in or near to a corner defined by an outer circumferential surface of the tube 11 and a rear end (an upwardly facing surface) 32b of the tube insertion hole 32 in the annular mounting fixture 31. Then, this product in process before brazing is placed in a vacuum oven so that the brazing filler material 220 is heated to melt the same (reflow). By so doing, the melted brazing filler is caused to flow into a gap defined between the inner circumferential surface of the tube insertion hole 32 in the annular mounting fixture 31 and the outer circumferential surface of the tube 11 for brazing. Also, the brazing filler material 220 may be applied to the location described above in the form of a paste.
[Patent Literature 1] JP-A-S62-278421